Synthesis of Molybdenum(VI) Neopentylidene Neopentylidyne Complexes

Article abstract of DOI:10.1021/acs.organomet.9b00412

Mo(C-t-Bu)(CH-t-Bu)(Cl)(PMe₂Ph)₂ (1) was prepared as off-white crystals in 26% yield through addition of 2.5 equiv of Mg(CH₂-t-Bu)₂ to Mo(O)[OC(CF₃)₃]₄ in diethyl ether followed by 3 equiv of PMe₂Ph and a workup that includes dichloromethane (the source of Cl). Compound 1 is largely a syn isomer initially that equilibrates to give approximately a 1:1 mixture of syn and anti isomers within 1-2 h. Compound 1 reacts with Li(3,5-dimethylpyrrolide) to give Mo(C-t-Bu)(CH-t-Bu)(η¹-Me₂Pyr)(PMe₂Ph)₂ (2a) as a pale yellow solid in 76% yield, and 2a reacts with Ph₃SiOH to give a mixture of syn and anti Mo(C-t-Bu)(CH-t-Bu)(OSiPh₃)(PMe₂Ph)₂ (3a) in 84% yield. All three compounds tend to lose PMe₂Ph to give 14e monophosphine complexes with the formulas Mo(C-t-Bu)(CH-t-Bu)(X)(PMe₂Ph) (X = Cl, Me₂Pyr, or OSiPh₃), none of which could be isolated. X-ray studies show the structures of 1, 2a, and 3a to be analogous with τ values of 0.45, 0.53, and 0.69, respectively.

Full text of DOI:10.1021/acs.organomet.9b00412

Communication
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Cite This: Organometallics XXXX, XXX, XXX−XXX
Synthesis of Molybdenum(VI) Neopentylidene Neopentylidyne
Complexes
†
Hosein Tafazolian,†,‡ Richard R. Schrock,*,†,‡ and Peter Muller
̈
†Department of Chemistry, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, United States
‡Department of Chemistry, University of California at Riverside, Riverside, California 92521, United States
S
* Supporting Information
ABSTRACT: Mo(C-t-Bu)(CH-t-Bu)(Cl)(PMe2Ph)2 (1) was prepared
as off-white crystals in 26% yield through addition of 2.5 equiv of
Mg(CH2-t-Bu)2 to Mo(O)[OC(CF3)3]4 in diethyl ether followed by 3
equiv of PMe2Ph and a workup that includes dichloromethane (the
source of Cl). Compound 1 is largely a syn isomer initially that
equilibrates to give approximately a 1:1 mixture of syn and anti isomers
within 1−2 h. Compound 1 reacts with Li(3,5-dimethylpyrrolide) to
give Mo(C-t-Bu)(CH-t-Bu)(η1-Me2Pyr)(PMe2Ph)2 (2a) as a pale
yellow solid in 76% yield, and 2a reacts with Ph3SiOH to give a mixture of syn and anti Mo(C-t-Bu)(CH-t-
Bu)(OSiPh3)(PMe2Ph)2 (3a) in 84% yield. All three compounds tend to lose PMe2Ph to give 14e monophosphine complexes
with the formulas Mo(C-t-Bu)(CH-t-Bu)(X)(PMe2Ph) (X = Cl, Me2Pyr, or OSiPh3), none of which could be isolated. X-ray
studies show the structures of 1, 2a, and 3a to be analogous with τ values of 0.45, 0.53, and 0.69, respectively.
mido alkyl complexes such as M(NR′)2(CH2R)2 (M = Mo
reagents.6 The final M(VI) complexes are formed only after
Ior W; R′ = aryl, tert-butyl, or adamantyl and CH R = exposure of the crude product to water and air, so it appears
2
likely that at least some, if not most, of the metal is reduced
and then reoxidized in the presence of water and air.
WO2(CH2-t-Bu)2(bipy) is a precursor to W(O)(CH-t-Bu)-
Cl2L2 upon reaction of WO2(CH2-t-Bu)2(bipy) with Me3SiCl,
ZnCl2, and 2 equiv of L (e.g., L = PPhMe2); W(O)(CH2-t-
Bu)2Cl2(bipy) is a plausible intermediate.7 This approach to
the synthesis of Mo(O)(CH-t-Bu)Cl2L2 complexes failed in
our hands so far. A second exception is the reaction between
Mo(O)(Cl)2(t-Bu3PN)2 and 2 equiv of LiCH2SiMe3 to give
Mo(O)(CHSiMe3)(t-Bu3PN)2 in 87% yield;8 Mo(O)-
(CH2SiMe3)2(t-Bu3PN)2 is the plausible intermediate in
this reaction. Unfortunately, Mo(O)(CHSiMe3)(t-Bu3PN)2
and analogous molybdenum imido alkylidene complexes that
contain two t-Bu3SiO− ligands (which are sterically approx-
imately equivalent to t-Bu3PN−ligands)9 are relatively
unreactive in olefin metathesis reactions.
neopentyl or neophyl), which can be prepared readily from
M(NR′)2(dme)Cl2 complexes and magnesium or lithium
alkyls, yield M(NR′)(CHR)X2 complexes when one of the
imido ligands is protonated with 2 equiv of HX (e.g., X =
triflate).1 However, analogous approaches to oxo alkylidene
complexes usually are thwarted by the fact that oxo ligands are
attacked by the alkylating agent and removed from the metal.
Attempted alkylations of Mo oxo complexes more often give
rise to low yields of oxo products, or none at all. For example,
Osborn reported the synthesis of Mo(O)(CH2-t-Bu)3Cl and
“Mo(O)(CH2-t-Bu)4″ through addition of Mg(CH2-t-Bu)2 to
Mo(O)Cl4,2 but experimental details (including yields) were
not provided in either of the succinct reports, and Mo(O)-
(CH2-t-Bu)4 had to be reformulated (as noted in footnote 1 in
ref 2b, without further details) as Mo(O)(CH2-t-Bu)3(OCH2-
t-Bu). The neopentoxide in Mo(O)(CH2-t-Bu)3(OCH2-t-Bu)
could arise through attack by the alkylating agent on the oxo
ligand.3 It also has been reported that addition of 6 equiv of (t-
BuCH2)MgCl to MoO2Cl2 in diethyl ether gives the best
reported yield (∼35%) of Mo(C-t-Bu)(CH2-t-Bu)3.4 The
analogous trimethylsilylmethylidyne complex, Mo(CSiMe3)-
(CH2SiMe3)3 (8% yield; liquid at 22 °C), has been prepared
similarly with (Me3SiCH2)3MoMo(CH2SiMe3)3 (25% yield;
crystalline) and unstable (Me3SiCH2)3MoCHSiMe3 (pro-
posed) being two other metal-containing products.5
We recently prepared molybdenum oxo alkylidene com-
plexes through a controlled addition of 1 equiv of water to
molybdenum(VI) OC(CF3)3 or OC(CF3)2Me benzylidyne
complexes.10 In the hope that more direct routes to these Mo
oxo alkylidene complexes could be successful, we turned to
alkylation of Mo(O)[OC(CF3)3]4.11 Yellow Mo(O)[OC-
(CF3)3]4 can be prepared readily from Mo(O)Cl4 and 4
equiv of NaOC(CF3)3. It can be sublimed under a good
vacuum (<10−2 mm) at 60−80 °C. It is poorly soluble in
organic solvents (pentane, benzene, toluene) but partially
soluble in diethyl ether or dichloromethane.
One exception to low yield alkylations of Mo oxo complexes
is the class of molybdenum or tungsten compounds with the
formula MO2(CH2R)2(bipy) (CH2R can be a variety of alkyls,
including neopentyl), which can be prepared through
alkylation of MO2Cl2(bipy) compounds with Grignard
Received: June 19, 2019
© XXXX American Chemical Society
A
DOI: 10.1021/acs.organomet.9b00412
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Communication
We decided to explore alkylation of Mo(O)[OC(CF3)3]4
with Mg(CH2-t-Bu)2 in the presence of PMe2Ph in order to
trap any monometallic products. Alkylation reactions using 0.5,
1.0, 2.0, and 2.5 equiv of Mg(CH2CMe3)2 in the presence of
PMe2Ph in diethyl ether were explored initially. Each crude
sample was dissolved in C6D6 or CD2Cl2 and examined by
proton and/or 31P NMR. A small multiplet in the region
characteristic of a molybdenum alkylidene having two
phosphines bound to the metal in proton NMR spectra in
CD2Cl2 grew slightly over a day, but no other product could be
identified. When 2.5 equiv of Mg(CH2CMe3)2 were used and
CH2Cl2 was part of the workup, the alkylidene could be
extracted into pentane and crystallized reproducibly from a
concentrated pentane solution at −20 °C as off-white needles
in a yield of 26%. When the crude brown residue was dissolved
in C6D6 and not exposed to dichloromethane, no alkylidene
resonance was observed, even after addition of 2 equiv of
PMe2Ph. However, when a drop of CD2Cl2 was added to this
sample, the alkylidene resonance grew in with time. The syn
alkylidene resonance can be observed in the 1H NMR
spectrum 5 h after addition of CD2Cl2 and stops increasing
Table 1. Selected Bond Lengths and Angles in 1, 2a, and 3a
1
2a
3a
Mo1−C1
Mo1−C6
Mo1−P1
Mo1−P2
1.691(3)
1.985(3)
2.5089(3)
2.5196(3)
166.80
152.57
160.28
104.33
2.5197(3)
1.661(11)
2.000(9)
2.5092(10)
2.5092(10)
172.59
151.52
164.97
102.49
2.227(3)
1.764(6)
1.9214(14)
2.5202(3)
2.5366(4)
170.61
152.16
171.25
103.51
2.0748(9)
Mo1−C1−t-Bu
Mo1−C6−t-Bu
P1−Mo1−P2
C1−Mo1−C6
a
Mo1−X
a
X = Cl (in 1), N (in 2a), or O (in 3a).
dichloromethane, and we also do not want to propose any for
forming 1 at this stage.
Compound 1 is related to tantalum bisneopentylidene
complexes (e.g., Ta(CH-t-Bu)2(PMe3)2Cl) which contain two
different neopentylidene ligands, one of which has a
significantly greater agostic interaction of its alkylidene CHα
bond with the metal than does the other.14 It also is related to
W(C-t-Bu)(CH-t-Bu)(CH2-t-Bu)(PMe3)2,15 square pyramidal
W(C-t-Bu)(CH-t-Bu)(CH2-t-Bu)(dmpe),15 and trimethylsilyl
relatives.16 The alkylidene is in the syn orientation in the
crystal chosen for the X-ray study. Compound 1 is stable in
benzene solution for several days at temperatures as high as 80
°C. Attempts to remove one of the phosphines from a C6D6
solution of 1 through addition of 1 equiv of B(C6F5)3 to 1 in
0.5 mL of C6D6 (0.04 M) led only to decomposition to
unidentified products. We propose that loss of phosphine in
the solid state at 1 atm is the reason for a failure to obtain
satisfactory elemental analyses for 1 and related derivatives 2a
and 3a described below.
1
after ∼60 h. There was no further change in the H NMR
spectrum.
An X-ray structural study confirms that 1 is the “Ene/Yne”
complex, Mo(C-t-Bu)(CH-t-Bu)(PPhMe2)2Cl (1; eq 1 and
Proton NMR spectra of 1 usually show two triplet alkylidene
1
resonances at 13.87 ppm (anti, JCH = 137.0 Hz) and 12.26
1
ppm (syn, JCH = 100.6 Hz). Freshly isolated 1 is usually syn-
rich, occasionally as high as 95% syn (see SI). Upon
recrystallizations of 1, the syn/anti ratio approaches 1:1 and
remains so.
Compound 1 reacts with Li(2,5-dimethylpyrrolide) to yield
the monopyrrolide complex, 2a (eq 2). Compound 2a can be
crystallized from pentane as off-white needles in 76% yield. An
X-ray study showed the structure of 2a (Figure 2) to be
analogous to that of 1, with the pyrrolide bound to the metal in
an η1 fashion. The overall geometry and bond lengths and
angles in 2a are similar to those found in 1 (Table 1).
Figure 1. Structure of 1 (τ = 0.45).
Figure 1), not an oxo alkylidene complex. The overall
geometry is approximately halfway between a TBP and an
SP (τ12 = 0.45), although 1 (and related derivatives described
below) will be drawn as a TBP with L in the apical positions
for convenience. The bond lengths and angles (Table 1) are
not unusual for neopentylidyne ligands (1.691(3) Å, 166.80°)
or syn neopentylidene ligands (1.985(3) Å, 152.57°). The
chloride must come from dichloromethane, a phenomenon
that has been reported in the literature for certain Mo13a,b and
Re13c compounds. No mechanistic details have been reported
for formation of chloride complexes in the presence of
In 1H NMR spectra of pure samples of 2a, two syn
alkylidene resonances are found, one triplet (at 11.65 ppm in
C6D6) for 2a and one doublet (∼10% of the total at 13.48 ppm
in C6D6). We ascribe the latter to a monophosphine complex,
Mo(C-t-Bu)(CH-t-Bu)(PPhMe2)(Me2Py) (2b). Compounds
2a and 2b (plus phosphine) are in equilibrium at room
temperature; in a sample of 0.033 M 2a in C6D6, the Keq was
1
found by H NMR spectroscopy to be 3.7 × 10−4 M at room
temperature. The 31P NMR of a sample of 2a in C6D6 at room
temperature shows three resonances at 30.4, 7.3, and −46.5
B
DOI: 10.1021/acs.organomet.9b00412
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Communication
doublet alkylidene resonance characteristic of a mono-
1
phosphine adduct can be detected by H NMR in this case,
even at temperatures up to 70 °C. Nevertheless, one of the
phosphines is labile enough to be scavenged as a borane adduct
upon addition of 1 equiv of B(C6F5)3 to 3a in C6D6. The
resulting monophosphine adduct shows a characteristic syn
1
alkylidene resonance in the H NMR spectrum at 13.32 ppm
(d, JHP = 3.8 Hz, JCH = 106.1 Hz). To date, we have not been
able to isolate 3b.
An exploratory NMR-scale ROMP reaction showed that
cyclooctene was polymerized by 3a (1%) in C6D6 upon
addition of B(C6F5) (1.1 equiv). The amount of cycloctene
consumed was 69% in 1 h, 76% in 4 h, and 87% in 24 h.
We will seek higher yield routes to 1 and related “Ene/Yne”
complexes, preferably those that do not require removal of oxo
ligands from the metal and/or scavenging of chloride from
dichloromethane. We are especially interested in 14e “stereo-
genic at metal” Mo(CR)(CH-t-Bu)(L)X complexes that
contain a single donor (L) and anionic X, e.g., 2b and 3b
and analogs. These are 14e relatives of (largely) imido
alkylidene1,17 and oxo7,10 alkylidene complexes that have
been explored to date as metathesis initiators. An important
question is whether an alkylidyne ligand (CR) can survive
unchanged in a sustained metathesis reaction, a possibility that
has not been addressed to our knowledge.
Figure 2. Structure of 2a (τ = 0.53).
ppm for 2b, 2a, and free phosphine, respectively. The
magnetization exchange rates between these two species
were measured through a series of 1D EXSY experiments at
different temperatures (>40 °C). For dissociation of phosphine
from 2b, the ΔH‡ was found to be 121 ( 8) kJ/mol and ΔS‡
was found to be 0.095 ( 0.026) kJ/mol K. For the reverse
reaction, ΔH‡ and ΔS‡ were found to be 76( 5) kJ/mol and
−0.028 ( 0.017) kJ/mol K, respectively. 2b was also the major
product in an NMR-scale reaction in C6D6 of 2a with 1 equiv
of B(C6F5)3 (see SI).
ASSOCIATED CONTENT
* Supporting Information
■
Compound 2a reacts cleanly with 1 equiv of triphenylsilanol
to generate 3a (eq 3). An X-ray study shows the structure of 3a
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.organo-
met.9b00412.
Detailed NMR data and spectra for all compounds and
details of X-ray studies (PDF)
Accession Codes
CCDC 1922920−1922922 contain the supplementary crys-
tallographic data for this paper. These data can be obtained
free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by
emailing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
to be analogous to the structures of 1 and 2a (Table 1 and
Figure 3). An alkylidene multiplet can be observed in the
proton NMR spectra of 3a at 13.29 ppm (in C6D6) with a 1JCH
of 100.6 Hz that is characteristic of a syn alkylidene. No
AUTHOR INFORMATION
Corresponding Author
*E-mail: rrs@mit.edu.
■
ORCID
Richard R. Schrock: 0000-0001-5827-3552
Author Contributions
H.T. performed all synthetic work while P.M. performed all X-
ray structural studies.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We are grateful for financial support from the National Science
Foundation (CHE-1463707) and the University of California
at Riverside.
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Figure 3. Structure of 3a (τ = 0.69).
C
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DOI: 10.1021/acs.organomet.9b00412
Organometallics XXXX, XXX, XXX−XXX